Compressor and/or expander device

ABSTRACT

Systems and methods for operating a hydraulically actuated device/system are described herein. For example, systems and methods for the compression and/or expansion of gas can include at least one pressure vessel defining an interior region for retaining at least one of a volume of liquid or a volume of gas and an actuator coupled to and in fluid communication with the pressure vessel. The actuator can have a first mode of operation in which a volume of liquid disposed within the pressure vessel is moved to compress and move gas out of the pressure vessel. The actuator can have a second mode of operation in which a volume of liquid disposed within the pressure vessel is moved by an expanding gas entering the pressure vessel. The system can further include a heat transfer device configured to transfer heat to or from the at least one of a volume of liquid or a volume of gas retained by the pressure vessel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application Ser. No. 61/216,942, filed May 22, 2009, entitled“Compressor and/or Expander Device,” the disclosure of which is herebyincorporated by reference in its entirety.

BACKGROUND

The invention relates generally to systems, devices and methods for thecompression and/or expansion of a gas, such as air, and particularly toa device that includes features that allow heat exchange from and/or togas that is being compressed and/or expanded.

Traditionally, electric power plants have been sized to accommodate peakpower demand. Electric power plants can be constrained in terms of howquickly they can start-up and shut-down, and it is commonly infeasibleto completely shut-down a power plant. The combination of power outputconstraints, and start-up and shut-down constraints, restricts a powerplant's ability to optimally meet a fluctuating power demand. Theserestrictions may lead to increased green house gas emissions, increasedoverall fuel consumption, and/or to potentially higher operating costs,among other drawbacks. Augmenting a power plant with an energy storagesystem may create an ability to store power for later use, which mayallow a power plant to fulfill fluctuating consumer demand in a fashionthat minimizes these drawbacks.

An energy storage system may improve overall operating costs,reliability, and/or emissions profiles for electric power plants.Existing energy storage technologies, however, have drawbacks. By way ofexample, batteries, flywheels, capacitors and fuel cells may providefast response times and may be helpful to compensate for temporaryblackouts, but have limited energy storage capabilities and may becostly to implement. Installations of other larger capacity systems,such as pumped hydro systems, require particular geological formationsthat are not be available at all locations.

Intermittent electric power production sites, such as some wind farms,may have capacities that exceed transmission capabilities. Absentsuitable energy storage systems, such intermittent power productionsites may not be capable of operating at full capacity. Intermittentproduction sites may benefit from a storage system that can be sized tostore energy, when the production site is capable of producing energy atrates higher than may be transmitted. The energy that is stored may bereleased through the transmission lines when power produced by theintermittent site is lower than transmission line capacity.

Compressed air energy storage (CAES) systems are another known type ofsystem in limited use for storing energy in the form of compressed air.CAES systems may be used to store energy, in the form of compressed air,when electricity demand is low, typically during the night, and then torelease the energy when demand is high, typically during the day. Suchsystems include a compressor that operates, often at a constant speed,to compress air for storage. Turbines and turboexpanders, separate fromthe compressor, are typically used to expand compressed air to produceelectricity. Turbines and turboexpanders, however, often require thecompressed air to be provided at a relatively constant pressure, such asaround 35 atmospheres. Additionally or alternatively, air at pressureshigher than 35 atmospheres may need to be throttled prior to expansionin the turbine, causing additional losses that also reduce theefficiency of the system, and/or reduce the energy density that astorage structure may accommodate. Additionally, to increase electricalenergy produced per unit of air expanded through the turbine, compressedair in such systems is often pre-heated to elevated temperatures (e.g.,1,000° C.) prior to expansion by burning fossil fuels that increases thecost of storing energy, reduces overall efficiency, and producesemissions associated with the storage of energy.

Known CAES-type systems for storing energy as compressed air have amulti-stage compressor that may include intercoolers that cool airbetween stages of compression and/or after coolers that cool air aftercompression. In such a system, for intercoolers to work efficiently,however, the air must still achieve substantial temperatures during eachstage of compression, prior to being cooled, which will introduceinefficiencies in the system. Thus, there is a need to provide for CAEStype systems that have improved efficiencies.

SUMMARY OF THE INVENTION

Systems and methods for operating a hydraulically actuated device/systemare described herein. In one embodiment, a system includes at least onepressure vessel defining an interior region for retaining at least oneof a volume of liquid or a volume of gas and an actuator coupled to andin fluid communication with the pressure vessel. The actuator can have afirst mode of operation in which a volume of liquid disposed within thepressure vessel is moved to compress and move gas out of the pressurevessel. The actuator can have a second mode of operation in which avolume of liquid disposed within the pressure vessel is moved by anexpanding gas entering the pressure vessel. The system can furtherinclude a heat transfer device configured to transfer heat to or fromthe at least one of a volume of liquid or a volume of gas retained bythe pressure vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an air compression and expansionenergy system according to an embodiment.

FIG. 2A is a schematic illustration of an air compression and expansionenergy system showing the flow of energy during a compression cycle,according to one embodiment

FIG. 2B is a schematic illustration of an air compression and expansionenergy system showing the flow of energy during an expansion cycle,according to one embodiment.

FIG. 3A shows a single stage of one embodiment of a compressor/expanderdevice.

FIG. 3B is a cross-sectional view of one divider, taken alongcross-section 3B-3B of FIG. 3A, and shows a schematic representation ofaverage, minimum distance between points within the air of a pressurevessel and surfaces within the pressure vessel through which heat is tobe transferred.

FIGS. 4A-4C show cross-sections of various configurations of dividersthat may increase heat transfer surface areas within a pressure vessel.

FIGS. 5A-5C show the air/liquid interface in different stages of acompression or expansion cycle, according to one embodiment.

FIG. 6 shows a vessel with a heat exchanger that may be used to transferheat to or from the liquid of a pressure vessel, according to oneembodiment.

FIG. 7A shows a multi-stage compressor/expander device, according to oneembodiment.

FIGS. 7B-7E show the multi-stage compressor/expander device of FIG. 7Ain various stages during a compression cycle.

FIGS. 7F-7I show the multi-stage compressor/expander device of FIG. 7Ain various stages during an expansion cycle.

FIG. 8 shows a compressed air storage system incorporated into a windturbine, according to one embodiment.

FIG. 9 shows a schematic, cross-sectional view of a compressor/expanderdevice configured so that it may be incorporated into a tower of a windturbine, according to one embodiment.

FIG. 10 shows a graph of pressure levels at different stages duringexpansion through a compressor/expander device for varying storagestructure air pressures, according to one embodiment.

DETAILED DESCRIPTION

System and methods to store energy as a compressed gas, such as air,and/or generate energy from stored, compressed gas, at improvedefficiencies are disclosed herein. Aspects of the device may relate toimprovements in thermodynamic and/or mechanical efficiency during thecompression of air and during the expansion of air.

The energy flow characteristics of air compression consist of acombination of various energy flows, including “work energy flow” and“heat energy flow”. Those familiar with the art will understand thedistinction between the terms: “energy”, “work”, “heat”, “temperature”,“pressure”, “volume”, and “density”. This discussion proceeds by usingthese terms in their thermodynamically-exact sense, but does not take-upteaching the distinction.

A well-known gas compression dynamic is that a gas, such as air,increases in temperature when it is compressed. The thermodynamicconcepts of heat and temperature interrelate such that a gas compressionprocess in which no heat flows out of the compressing gas, results inthe maximum gas temperature increase. Such a zero heat flow process, isknown as an “adiabatic” process. In contrast, if heat flows out ofcompressing gas at a sufficient rate, the gas may compress with nochange in temperature. Such a constant temperature process is known as“isothermal” compression.

For a given gas volume reduction, an adiabatic compression processresults in the highest gas pressure, the highest gas temperature, andthe highest work consumption. In contrast, for the same volumereduction, an isothermal compression process results in the lowestpressure, lowest gas temperature (i.e. the same as the startingtemperature), and lowest work consumption. Processes that involveintermediate levels of heat flow, result in intermediate values of gaspressure, gas temperature, and work consumption. Those skilled in theart will recognize that a perfectly isothermal air compression processis a theoretical extreme that can only be achieved in reality byinvolving a relatively cold heat sink; regardless it is a useful metricfor air compression/expansion discussion and analysis.

Because it may affect pressure, temperature, and work, the ability toapproach an isothermal gas compression process may be useful fordesigning an energy storage device. A fundamental goal for a compressedair energy storage device, is minimizing the work consumed to achieve acertain gas storage condition; defined by the gas's density,temperature, pressure, and volume. While minimizing the work consumedduring gas compression is a fundamental goal of an energy storagedevice, those familiar with the art will recognize the need to attend tothe energy flows related to heat; both during compression, and duringstorage. Moreover, those familiar with the art of machine design willrecognize the need to attend to constraints related to pressure andtemperature; and will recognize the benefits that may result from lowertemperatures and pressures. Those familiar with the art ofthermodynamics will recognize that the factors related to gascompression, relate in inverse fashion to gas expansion, and therebypertain to extracting energy from expanding gas. With respect to anenergy storage system, those familiar with the art of thermodynamics andmachine design, will recognize, that an isothermal gas compressionprocess alone, is not sufficient to achieve a useful energy storagesystem, but will also recognize the enabling benefits a near-isothermalprocess presents.

The work involved with attaining a particular pressure in thecompression of air may be reduced by removal of heat from the air duringthe compression process, decreasing the extra work required as a resultof the pressure increase from a rise in temperature. Similarly, theamount of work that can be derived from compressed air, as the airexpands to a given pressure, can be increased by the continuous additionof heat preventing the air temperature from dropping during theexpansion process.

Heat (i.e., thermal energy) may be removed from air during compression.Removing heat in this manner may reduce the maximum temperature that asystem may be designed to accommodate. Additionally, increasing densityat a given pressure and removing heat from air may increase the mass ofair that can be stored in a given volume of space, and reduce the workrequired to increase the density of the air at the storage pressure. Itis to be appreciated that a given mass of air occupies less space whenat a lower temperature. In this regard, providing relatively cooler airto a storage device may increase the total mass of air that may bestored by the system.

Heat may also be removed prior to or during the intake stroke whichrealizes a number of benefits including higher density air at thebeginning of the compression stroke, and drying of humid air. Thisaction is also achieved by exposure of the air during the intake stroketo the heat capacitor structure that has been cooled by the liquidduring the preceding compression stroke. In addition, a pre-coolerupstream of the intake can achieve a similar or additional benefit.

Additionally, thermal energy may be added back to the expanding air toraise or maintain its temperature at any time prior to discharging theair to the atmosphere. Adding heat to the compressed air raises thepressure over what it would otherwise be. In this manner, the system canoutput the same or greater power with a smaller mass flow of air fromstorage. In other words, more power for the same mass flow.

In some embodiments, one or more features that promote greater heattransfer during compression and/or expansion are provided. Such featuresmay include, but are not limited to, a relatively slow compressionand/or expansion cycle, a relatively large heat transfer area for agiven volume of air between the air and adjacent surfaces, and/or a lowaverage minimum distance between air in a device and the liquid orstructure of the device through which heat is transferred.

In some embodiments, a system includes a compressor/expander device thatmay be used to compress air, in one operating mode, for storage in astorage structure. The compressed air may be expanded, at a later time,through the same compressor/expander device in a different operatingmode to release energy. Heat may be removed from the air duringcompression and/or added to the air during expansion to improveefficiencies of the device. Roundtrip thermal efficiencies (i.e.,efficiencies associated directly with the compressing an amount of airand then later expanding the same amount of air to produce mechanicalenergy, exclusive of mechanical, electrical, or other parasitic systemlosses) may be 50% or higher, 60% or higher, 70% or higher, 80% orhigher, and even 90% or higher.

In some embodiments, a compressor/expander device can include one ormore pressure vessels that are to be at least partially filled with aliquid during at least a portion of a compression and/or expansioncycle. In a compression mode of operation, air can be drawn into thepressure vessel from the atmosphere or received from an upstreamcompressor as an actuator of the device displaces the liquid from withinthe vessel to increase the volume available for air in the pressurevessel. The liquid is then moved or pumped into the vessel by theactuator to reduce the volume available for air in the pressure vesselto compress and deliver the air therefrom. In an expansion mode ofoperation, pressurized air may be received by a pressure vessel todisplace the liquid therein and drive the actuator to release andtransfer energy from the compressed air. Air that has been expanded maythen be discharged from the pressure vessel to the atmosphere, to adownstream compressor/expander device or other device for furtherexpansion as the volume available for air in the pressure vessel is thendecreased.

In some embodiments, heat may be transferred from air that is compressedin the pressure vessel to reduce the work required to achieve a givendensity, which may increase the efficiency of the compression process.In some embodiments, a device that may provide for increased heattransfer include, but is not limited to, a relatively slow operatingspeed at which compression and/or expansion may occur. In someembodiments, a complete compression or expansion cycle may be slowenough to provide additional time for heat transfer between the air andliquid. Enough heat energy may be transferred, according to someembodiments, to approximate an isothermal compression and/or expansionprocess, achieving efficiencies associated therewith. Additionally oralternatively, faster speeds may allow larger power levels to beachieved during expansion, isothermally or with temperature changes,which may be desirable at times to system operation.

While recognizing that attending to energy flows is fundamental todesigning an energy storage system, to be useful, it is also importantfor the system to achieve meaningful energy flow rates. Energy flowrate, meaning energy per unit time, is also known as “power”. The valueof meaningfully high power levels should be clear those skilled in theart. It bears pointing out, however, that a key aspect of the describedinvention is the heat flow rate it may achieve betweencompressing/expanding air, and the system's process liquid. Moreover,the key feature that the invention achieves may be the relatively highheat transfer rates it achieves in response to relatively small airtemperature changes.

In some embodiments, heat may be transferred from and/or to air that iscompressed and/or expanded through liquid that is present in a pressurevessel. As is to be appreciated, an air/liquid interface may move and/orchange shape during a compression and/or expansion process in a pressurevessel. This movement and/or shape change may provide acompressor/expander device with a heat transfer surface that canaccommodate the changing shape of the internal areas of a pressurevessel through which heat is transferred during compression and/orexpansion. In some embodiments, the liquid may allow the volume of airremaining in a pressure vessel after compression to be nearly eliminatedor completely eliminated (i.e., zero clearance volume).

Generally speaking, a liquid may have a relatively high thermalcapacitance as compared to air such that the liquid may maintain arelatively constant temperature as heat is passed therethrough,buffering the system from substantial temperature changes. Heat that istransferred between the air and liquid, or components of the vesselitself, may be moved from or to the pressure vessel through one or moreheat exchangers that are in contact with the liquid or components of thevessel. One type of heat exchanger that may be used to accomplish thisis a heat pipe, as discussed in greater detail herein. Thermal controlof the air and process liquid may be accomplished by mass transfer, heattransfer or any combination of the two.

In some embodiments, dividers may be positioned inside the volume of apressure vessel to increase the heat transfer area at heat transfersurfaces, both liquid and solid, of the pressure vessel and air that isbeing compressed and/or expanded. Methods to increase heat transfersurface area contemplated include the use of fluid to solid and fluid tofluid. Each of the dividers may be shaped and/or may be positioned totrap a volume or pocket of air within a pressure vessel that providesone or more air/liquid interfaces in addition to an interface betweenthe divider and the air (i.e., air/divider interface). The air/liquidinterfaces and air/divider interfaces provide surfaces across which heatmay be transferred during compression and/or expansion. The dividers maybe configured such that the area of the liquid through which heat istransferred, whether directly at air/liquid interfaces or indirectlythrough portions of a divider at air/divider interfaces, may remainsubstantially constant, even toward the end of a compression cycle, whenonly small volumes of air may remain in a pressure vessel. Maintaininglarge surface areas for heat transfer toward the end of compression mayimprove efficiency during compression, as this portion of thecompression process, absent heat removal, typically experiences thegreatest rise in temperature and greatest impairment to compressionefficiency. It is to be appreciated that, toward the end of compression,an incremental change in the volume available for air may cause thegreatest percent change in the overall volume that is available for air,and consequently, may be associated with a greater change intemperature, absent heat removal. Similar effects may be realized bymaintaining a relatively large area for heat transfer to air from liquidand/or the dividers throughout and particularly at the beginning of anexpansion cycle.

In some embodiments, dividers that provide an air/liquid interface and aair/divider interface for heat transfer to/from the air may allowstructural components of the pressure vessel (e.g., the exterior shell)to be shaped and/or sized for optimal structural and/or shippingconstraints, while also increasing areas for heat transfer with air thatis being compressed or expanded within the pressure vessel. According tosome embodiments, the dividers may include a dish-like or other openended shape(s) configured to hold a pocket of air within the pressurevessel as air is compressed and/or expanded. The dividers may bearranged to have an opening that faces downwardly to channel the flow ofair (i.e., toward the direction in which gravity pulls) when thepressure vessel is oriented in its operational position to help holdpockets of air in contact with liquid also in the pressure vessel.

In some embodiments, dividers that hold pockets of air within a pressurevessel may provide for a reduced average minimum distance between pointswithin the air volume and surfaces in contact with the air from whichheat is received or transferred. In some embodiments, the dividers maybe arranged in a stack configuration of dish-like structures that trappockets of air formed as relatively thin layers and that provide a smallaverage minimum distance between points of an air pocket and surfaces incontact with the air. Reducing the average minimum distance, in thisrespect, reduces the average distance that heat may have to travel,whether through conduction or convection, to or from the air pocket,which may have a higher thermal resistivity than materials across whichheat may travel during compression and/or expansion, including liquid inthe pressure vessel or the metal of the pressure vessel itself.

In some embodiments, a compressor/expander device can allow a system toachieve efficiencies equal to or greater than those associated withexisting compressed air energy storage (CAES) systems with only the useof low-grade heat sources and/or heat sinks (e.g., heat sourcestypically at temperatures between about 10° C. to 50° C., among otherranges, and heat sinks that are typically at lower ranges oftemperatures) and without requiring the energy input associated withfuel that may otherwise be used to heat air during expansion, as in atraditional CAES system. Eliminating or reducing the need to burn fuelto heat air at expansion may allow the compressor/expander device tooperate without the production of emissions, or at least without theproduction of emissions associated directly with the storage and releaseof energy as compressed air.

A compressor/expander device as described herein can be configured suchthat movement of a single actuator causes compression of air in a firstpressure vessel of the device and also allows for the simultaneousreceipt of air in a second pressure vessel of a common stage and thatoperates in coordination with the first pressure vessel, when operatedin a compression mode. In this manner, the actuator may be a doubleacting device. Similarly, expansion and discharge of air may occur inthe first and second pressure vessels, alternately, as an actuator movesback and forth between pressure vessels of a common stage. Additionallyor alternatively, compressor/expander devices may be configured inseries to form a multi-stage device to help achieve greater airpressures, such as up to 150 psi or greater after a first stage, up to1,000 psi or greater after a second stage, and/or up to 3,000 psi orgreater after a third stage, at improved efficiencies.

A compressor/expander device as described herein can also allowcompression and/or expansion to occur across different stages of amulti-stage compressor/expander device; for example, during expansion,intake in one (smaller vessel) while discharge in the other (largervessel). By way of example, a device may include an upstream pressurevessel (e.g., a first pressure vessel of a first stage) and a downstreampressure vessel (e.g., a first pressure vessel of a second stage) inwhich air may be compressed at a common time. A change in volumeavailable for air that occurs in the downstream pressure vessel may beless than a change in the volume available for air in the upstreampressure vessel. At the beginning of compression, the volume availablefor air in each of the upstream pressure vessel and the downstreampressure vessel may be in fluid communication with one another.Additionally, the volume available for air in the downstream pressurevessel may be at a minimum value while the volume available for air inthe upstream pressure vessel is at a maximum value. Compression of airmay occur in the combined volumes of the upstream pressure vessel andthe downstream vessel as the volume available for air in the upstreampressure vessel decreases. The reduction in the volume available for airin the upstream pressure vessel may result in the compression of air,despite an increase in the volume available for air in the downstreampressure vessel since a reduction in the volume available for air in theupstream pressure vessel is greater than an increase in the volumeavailable for air in the downstream pressure vessel.

Embodiments of the compressor/expander device may operate at relativelylow speeds, as discussed above, which may result in lower operatingtemperatures for the device. Lower temperatures and slower speeds atfriction surfaces may extend the wear life and/or lend to increaseddevice reliability.

A compressor/expander device may accommodate varying input power levels,as may be associated with wind farms having power outputs that depend onwind levels. According to some embodiments, the compressor/expanderdevice may be a positive displacement device that, unlike centrifugalcompressors found in some CAES systems, may efficiently operate over awide range of speeds or output levels.

A compressor/expander device may also allow for a constant power outputfor varying compressed air pressure levels of a storage structure.Valves, sensors, and/or other control devices may be incorporated into acompressor/expander device to control a mass of air that is admitted tothe device for expansion, regardless of pressure level in a storagestructure. In this respect, an amount of energy produced by the devicemay be maintained relatively constant. Additionally or alternatively,the mass of air admitted to the compressor/expander device may beincreased/decreased, when desired and when storage structure pressurelevels permit, such that additional/reduced power may be produced. Rateof compression/expansion can be varied by the amount of air taken in orthe speed of the stroke or both.

A compressor/expander device may be constructed modularly to allow aplurality of devices to be sized together relatively easily fordifferent applications. According to some embodiments, individualcompressor/expander devices may be sized for power ranges between 1.0megawatts and 5.0 megawatts, although other sizes are possible. Use of aprecompressor in-line before the compressor may also be employed toprovide initial compression of the air. Multiple compressor/expanderdevices may be operated in parallel to provide larger power capacities.By way of example, according to one embodiment, one hundred and fifty,2.0 megawatt devices may be operated in parallel to provide for a 300megawatt installation. If desired, fewer than the full complement of onehundred and fifty compressor/expander devices may be in operation, withthe remaining devices remaining idle, to provide for efficient systemoperation at varying power levels. Additionally or alternatively,installations of multiple compressor/expander devices may beginoperation with less than the full complement of planned devicesinstalled to allow a system to be at least partially operational priorto the system being constructed completely.

FIG. 1 is a schematic illustration of an embodiment of an energy system100 in which a compressor/expander device may be used to both storeenergy and release energy that has previously been stored. As shown inFIG. 1, a wind farm 102 including a plurality of wind turbines 104 maybe used to harvest and convert wind energy to electric energy fordelivery to a motor/alternator 110. It is to be appreciated that thesystem may be used with electric sources other than wind farms, such as,for example, with the electric power grid, or solar power sources. Themotor/alternator 110 drives an actuator 112 connected to acompressor/expander device 120.

Energy can be stored within the system 100 in a compressed form and thenexpanded for use at a later time period. To store energy generated bythe wind farm 102, the actuator 112 uses a hydraulic pump (not shown inFIG. 1) to cause liquid in a pressure vessel (not shown in FIG. 1) ofthe compressor/expander 120 to move or be displaced to increase a volumeavailable within the pressure vessel for the receipt of air. Theactuator 112 then compresses the air by causing liquid in the pressurevessel to move or be displaced to decrease the volume available for airin the pressure vessel. During this process, heat is removed from theair. During compression, the air is delivered to a downstream stage ofthe compressor/expander device 120 and eventually at an elevatedpressure to a compressed air storage structure 122 (also referred toherein as “cavern”). At a subsequent time, for example, when there is arelatively high demand for power on the power grid, or when energyprices are high, compressed air may be released from the storagestructure 122 and expanded through the compressor/expander device 120.Expansion of the compressed air drives the actuator 112 that, in turn,drives the motor/alternator 110 to produce electricity for delivery tothe power grid 124. Heat at a relatively low temperature (e.g., betweenfor example, about 10° C. and about 50° C.) may be added to the airduring expansion to increase the power generated during the expansionprocess.

FIG. 2A is a schematic illustration of energy flow through a multi-stagesystem 200 similar to the system 100 of FIG. 1, at one example operatingcondition as air is being compressed for storage. As described above, amotor/alternator 210 drives an actuator 212 which can use a hydraulicpump (not shown in FIG. 2A) to cause liquid in a pressure vessel (notshown in FIG. 2A) of the compressor/expander 220 to move or be displacedto increase a volume available within the pressure vessel for thereceipt of air. The actuator 212 then compresses the air by causingliquid in the pressure vessel to move or be displaced to decrease thevolume available for air in the pressure vessel.

Heat energy is removed during compression via a liquid that is presentin one or more pressure vessels (not shown) of a multi-stagecompressor/expander device 220 to maintain the air that is beingcompressed at a relatively constant temperature. The heat energy istransferred from the liquid and the compressor/expander device 220 to aheat sink via a heat exchanger. In another configuration, the heatenergy stays in the liquid, and the liquid is discharged out of thecompression chamber directly to a heat sink, where it discharges itsheat, and is then returned to the pressure vessel. The air may achievepressures of about, for example, 150 psi, 1,000 psi, and 3,000 psi ateach of first, second, and third stages before being delivered to astorage structure 222 at a pressure of about 3,000 psi, according to oneembodiment. The temperature of the air, after being provided to thecompressor/expander device 220, and initially compressed and cooled,remains relatively constant, such as, for example, at about 5° C., 10°C., 20° C., 30° C. or other temperatures that may be desirable, untildischarged to the storage structure 222. Air stored in the storagestructure 220 may be heated (or cooled) naturally through conductive,convective, and/or radiative heat transfer if the storage structure 222is naturally at a higher (or lower) temperature. For example, in somecases, the storage structure may be an underground structure, such as asalt cavern constructed in a salt dome or bedded salt layer that is/areused for storing the compressed air or an aboveground storage tank orvessel. In another embodiment, an above ground storage structure couldbe painted black and designed to facilitate absorption of solarradiation for heating. In another embodiment, a below ground storagefeature could take advantage of geothermal heat. It is to be appreciatedthat FIG. 2A illustrates one operating condition for one embodiment of asystem, and that other operating conditions exist and that other systemembodiments are also contemplated.

FIG. 2B is a schematic representation of energy flow through the system200 of FIG. 2 at one operating condition, as air is being released fromstorage for the production of energy. In one example operatingcondition, air in the storage structure 222 can be at about 3,000 psi,and can be expanded through the third, second, and first stages of thecompressor/expander device to gauge pressures of, for example, about1,000 psi, 150 psi, and 0 psi, respectively. Heat may be added to theair during expansion at each of the third, second, and first stages,respectively, to hold air temperatures at a substantially constanttemperature, such as at about 35° C. or other temperatures, during theentire expansion process. It is to be appreciated, that the overalltemperature change of air during expansion may be limited by arelatively large amount of air that expands in a relatively small volumeof a pressure vessel and that is in contact with substantial heattransfer surfaces. The compressor/expander device 220 producesmechanical power that is converted through one or more hydraulicpumps/motors of the actuator 212, and a motor/alternator 210 is used toproduce electric power. It is to be understood that actuators other thanhydraulic actuators can alternatively be used.

FIG. 3A illustrates a portion of a compressed air storage system 300that includes a compressor/expander device 320 and an actuator 312. Thecompressor/expander device 320 illustrates a single stage of acompressed air storage system. The compressor/expander device 320includes a first pressure vessel 324 and a second pressure vessel 326.The first and second pressure vessels 324, 326 are each coupled fluidlyto the actuator 312 by a conduit or housing 328 and 330, respectively.The actuator 312 can include a water pump that includes a hydraulicallydriven piston 332. The piston 332 is disposed within a housing orreservoir 340 and can be driven with one or more hydraulic pumps (notshown in FIG. 3A) to move toward and away from the conduit 328 of firstpressure vessel 324 to alternately reduce and then increase the internalair volume of the first pressure vessel 324 (with an equivalent, butopposite increase and reduction of air volume in the second pressurevessel 326). Each of the first and second pressure vessels 324, 326 areat least partially filled with a liquid, such as water, that is moved bythe actuator 312 to alternately compress and drive air from the volumeof each of the first and second pressure vessels 324, 326, when operatedin a compression mode, or to be moved by compressed air received ineither of the first and second pressure vessels 324, 326 when operatedin an expansion mode.

The compressor/expander device 320 can also include dividers 334 thatcan be positioned within the first and second pressure vessels 324, 326.The dividers 334 can increase the overall area within a pressure vesselthat is in direct or indirect contact with air, which can improve heattransfer. The dividers 334 can provide for an increased heat transferarea with both air that is being compressed and air that is beingexpanded (either through an air/liquid interface area or air/dividerinterface), while allowing the exterior structure and overall shape andsize of a pressure vessel to be optimized for other considerations, suchas pressure limits and/or shipping size limitations. It is to beappreciated that the dividers may heat up or cool down during eachcompression event, and that the water or liquid will thermally rechargethe dividers back to the temperature of the water during eachcompression or expansion event, allowing the dividers to act as arechargeable thermal capacitor. It is also to be appreciated that thedividers could have interior spaces that are occupied with a fluid suchas a refrigerant like water, propane, or other refrigerant, and therefrigerant could be cycled outside the compression/expansion chamber toa heat sink/source.

In this embodiment, the dividers 334 are arranged in a stackconfiguration within the first and second pressure vessels 324 and 326.Each divider 334 can be configured to retain a pocket of air. In oneillustrative embodiment, each of the dividers 334 can include an upperwall, a downwardly extending side-wall that may conform in shape andsubstantially in size to the inner wall of the pressure vessel, and anopen bottom. Various shapes of dividers may be used, as shown, forexample, in FIGS. 4A-4C, described in more detail below. The open bottomof each of the dividers 334 face in a common, substantially downwarddirection when the pressure vessel is oriented for operation. It is tobe appreciated that although the figures show dividers that conform insize and shape to the interior of the pressure vessels 324, 326, and aregenerally shaped similarly to one another, other configurations are alsopossible and contemplated, including embodiments that include dividersthat are substantially smaller in width than the interior of a pressurevessel and/or that are shaped and sized differently than one another,among other configurations. Some dividers can be used that do not faceany particular direction or contain a pocket of air. Such dividers maybe configured to minimize the distance that heat must travel through theair in order to reach the dividers, such as a maximum distance of ⅛ ofan inch, and other distances. Such configurations may include paralleldividers, corrugated dividers, intersecting dividers, curved dividers,dividers made out of concentric rings, dividers made out of pressedand/or stamped rolled or sheet metals, and many other shapes andconfigurations, some of which are or may be routinely used in variousthermal transfer devices. Various other shapes and configurations ofdividers can be used, such as, for example, the dividers that are shownand described in U.S. Provisional App. No. 61/290,107, entitled “Systemand Methods for Optimizing Efficiency of a Hydraulically ActuatedSystem,” incorporated herein by reference in its entirety.

As shown in FIG. 3A, a manifold 336 can extend centrally through thestack of dividers 334 and fluidly couple each of the dividers 334 to aninlet/outlet port 338 of the pressure vessels 324, 326. In otherembodiments, the manifold may include multiple tubes and/or may belocated peripherally about the stack of dividers or in other positions.Air may enter and/or exit the pressure vessels 324, 326 through theports 338, and can provide a conduit for fluid communication betweenpockets of air associated with each divider 334. In other embodiments,such as those in which dividers do not retain a pocket of air, themanifold may not be included.

The embodiment of FIG. 3A is one example of an arrangement of pressurevessels and an actuator that can be used within an air compression andstorage system. It should be understood, that other arrangements arealso possible and contemplated. By way of example, although the actuatoris shown as including a single, double acting piston that is orientedvertically, other embodiments may include housings with actuators thatinclude horizontally oriented pistons and/or multiple hydraulic pistonsthat operate in parallel and/or in series to move liquid within pressurevessels. According to some embodiments, actuators may lack pistonsaltogether, and instead comprise pumps that move liquid into and out ofthe pressure vessels. Multiple pumps and/or pistons can additionally oralternatively, be used in parallel to move liquid into and out ofpressure vessels, according to some embodiments. Still, according toother embodiments, an actuator, such as a hydraulic piston, may have adirect mechanical connection to the motor/alternator of the system, asembodiments of the system are not limited to that shown in the figures.

The dividers 334 in the embodiment of FIG. 3A can increase the area ofheat transfer surfaces that are in contact with air, includingair/liquid interface areas and air/divider interface areas, at pointsduring a compression and/or expansion according to the number ofdividers and/or the surface area of the dividers. The heat transfer fromthe air and/or liquid to the dividers is also affected by the mass ofthe dividers, their thermal capacitance, and/or their thermalconductivity. As is to be appreciated, the air/liquid interface, absentthe dividers, may be equal to the internal, horizontal cross-sectionalarea of the pressure vessel. Each of the dividers in the embodiment ofFIG. 3A provides an air/liquid interface and/or an air/divider interfacethat is substantially equal in size to the cross-sectional area of thepressure vessel. In this respect, the total area of the air/liquidand/or the air/divider interface may be increased, at any given timeduring expansion or compression, by a multiple substantially equal tothe number of dividers and/or the surface area of the dividers in thepressure vessel. Additionally, each of the dividers may provide anair/divider interface that is also substantially equal in size to thecross-sectional area of the pressure vessel. In this regard, pockets ofair associated with each divider may be substantially surrounded withliquid, either in direct contact with the liquid or in indirect contactwith the liquid through a surface of the divider, to increase areaavailable for heat transfer with the liquid and the air. According tosome embodiments, the number of dividers and/or the multiple by whichthe dividers increase the total area of the air/liquid interface and/orair/divider interface, at a particular time during compression and/orexpansion, may be 5 or higher, 10 or higher, 20 or higher, 30 or higher,40 or higher, or even 50 or higher. In other embodiments, the dividerswill be more tightly packed, and may be spaced so that in all or aportion of the pressure vessel, the dividers are separated from eachother by no more than 1 inch, ½ inch, ¼ inch, ⅛ inch, 1/16 inch, or someother number.

The dividers in the embodiment of FIG. 3A may, additionally oralternatively, maintain total heat transfer surface areas at high levelssubstantially throughout a compression and/or expansion cycle. Thedividers may be placed closer together toward the top of the pressurevessel in order to accommodate the increased thermal loads toward theend of a compression event and at the beginning of an expansion event.As is to be appreciated, the total surface area available for heattransfer during a compression and/or expansion process may include thesurface area of liquid and the divider that are in contact with the airthroughout a complete compression and/or expansion cycle. That is, thetotal surface area for heat transfer may include the area that is indirect contact with the air (either areas of the liquid or the divider)integrated over the time of a compression and/or expansion cycle. Inthis respect, configuring dividers to maintain an increased heattransfer surface throughout a compression and/or expansion cycle mayincrease the total area available for heat transfer, when considered asa time integral over a complete compression and/or expansion cycle, by amultiple much greater than the number of dividers that are present in apressure vessel.

Dividers positioned inside a pressure vessel may additionally oralternatively reduce the average minimum distance between points of airthat is to be compressed or expanded and the thermal conduction surfacesinside of a pressure vessel (either air/liquid interfaces or air/dividerinterfaces) through which heat is to be transferred. The dividers mayalso be textured, pocketed, stamped, coated, serrated, cut, bent,covered with a coating or layer of other material, or otherwise treatedto increase or decrease their surface area, increase or decrease theirability to stay wet or hold water, to increase or decrease turbulence inthe air or water, all to promote more effective heat transfer whileminimizing irreversible energy losses. FIG. 3B shows a cross sectionalview of a divider 334 that includes an air pocket 344, and the surfacesthrough which heat is to be transferred. As illustrated, the pocket 344may be a relatively thin layer of air under or within a divider 334. Anypoint within the pocket 344 is no further away from either the upperwall 346 of the divider 344 (i.e., the air/divider interface) or theliquid that is present in the divider 334 (i.e., the air/liquidinterface 348) than one half of the height H of the divider itself. Inthis respect, heat, when transferred in conduction, will only need totravel, at most, a distance equal to one half of the height H of thedivider to reach one of the air/liquid interface or the air/dividerinterface. Similarly, when transferred in convective modes, airmolecules may only need to travel, at most, a distance equal to one halfof the height H of the divider to reach one of the air/liquid interfaceor the air/divider interface for heat transfer to occur.

Minimizing the distance between air in the pressure vessel and surfacesthrough or into which heat is to be transferred may substantiallyimprove heat transfer to and/or from air that is compressed and/orexpanded.

Air typically has the lowest thermal conductivity among the mediathrough which heat is transferred in the compression/expansion device.By way of example, air has a thermal conductivity of about 0.024Watts/meter-Kelvin while water has a thermal conductivity that is anorder of magnitude greater that that of air (0.58 Watts/meter-Kelvin)and steel has thermal conductivity that is about three orders ofmagnitude greater than that of air (43 Watts/meter-Kelvin for 1% carbonsteel). Reducing the distance that heat travels through air essentiallyreduces the greatest bottleneck to heat transfer by reducing thedistance of the most thermally resistive element along the heat transferpath.

Dividers may be shaped differently than shown in the embodiment of FIG.3A and/or may be packaged within pressure vessels in differentarrangements. The dividers of FIGS. 3A and 3B are downwardly facing (andshaped substantially like inverted dishes), so as to form and trap airpockets therein.

It is to be appreciated that other shapes are possible, such as dividerswith domed upper walls, as shown in FIG. 4A, flared sidewalls as shownin FIG. 4B (either flared inwardly or outwardly), or other shapes, asembodiments are not limited to that which is shown in the figures.Additionally or alternatively, although the dividers of FIGS. 3A and 3Bare sized to occupy an area substantially equal to a cross-sectionalarea of the pressure vessel, smaller dividers are also possible.

In the illustrated embodiments of FIGS. 4A and 4B, a divider 434 isdisposed within a pressure vessel 426. The divider 434 includes a domedupper wall 446 and one or more passages 450 may be provided between eachof the pockets created by the dividers 434 and a manifold 436 to allowthe passage of air and/or liquid therebetween. It is also contemplatedthat fluid communication between the dividers 434 and manifold 436 maybe provided by different means, such as by manifolds that are positionedexternal to a pressure vessel and/or manifolds that are positionedoff-center within a pressure vessel.

FIG. 4B illustrates an embodiment that includes a divider 534 isdisposed within a pressure vessel 526. The divider 534 includes an upperwall 546 and one or more passages 550 may be provided between each ofthe pockets created by the dividers 534 and a manifold 536 to allow thepassage of air and/or liquid therebetween. The divider 534 also includesoutwardly flared side-walls 552.

Dividers may be configured to create a turbulent air/liquid interface tofurther increase the heat exchange between the air and liquid of adivider of between air and surfaces of the divider itself. By way ofexample, according to some embodiments turbulators may be positioned onthe interior of a divider to agitate liquid as the air/liquid interfacemoves upward or downward during compression and/or expansion modes,effectively increasing the air/liquid interface area and promotingconvective heat transfer to and/or from air. According to otherembodiments, such as shown in FIG. 4C, a divider 634 is shown disposedwithin a pressure vessel 626 and includes a bank of heat transfer fins654 that may be incorporated onto surfaces of the divider 634 to promoteheat transfer between an air pocket of a divider and the surfaces (e.g.,upper wall 646) of the divider. It is to be appreciated, however, thatnot all embodiments include turbulators or banks of fins, as the variousembodiments are not limited to that shown or explicitly describedherein.

As mentioned above, the size and shape of a pressure vessel may beoptimized for considerations other than the air/liquid interface areawhen a plurality of dividers are used to define the air/liquidinterface. By way of example, according to some embodiments, dividersmay allow the total area of the air/liquid interface to be maximizedwhile the overall size of the pressure vessel is designed to have amaximum outside dimension (i.e., the greatest of the length, width, andheight of a pressure vessel) below a particular distance, which mayprove useful when pressure vessels are to be packaged for shipmenteither separately or in an ISO standard shipping container. Additionallyor alternatively, pressure vessels may be shaped to provide for optimalstructural integrity, having cylindrical, spherical/cylindrical, orother shapes. According to some embodiments, the maximum dimension of acylindrical pressure vessel with a rounded top and bottom structure maybe about 6 meters while having a total air/liquid surface area of about140 square meters, a maximum dimension of about 2.5 square meters and atotal air/liquid surface area of about 40 square meters, or a maximumdimension of about 2 meters and a total air/liquid surface area of about10 square meters.

As discussed above, heat can be transferred from and/or to air that iscompressed and/or expanded by liquid (e.g., water) within a pressurevessel. An air/liquid or air/divider interface (e.g., provided in partby dividers discussed above) may move and/or change shape during acompression and/or expansion process in a pressure vessel. This movementand/or shape change may provide a compressor/expander device with a heattransfer surface that can accommodate the changing shape of the internalareas of a pressure vessel through which heat is transferred duringcompression and/or expansion. In some embodiments, the liquid may allowthe volume of air remaining in a pressure vessel after compression to benearly eliminated or completely eliminated (i.e., zero clearancevolume).

FIGS. 5A-5C show the air/liquid interface associated with a divider 734at various stages of compression and expansion. At the beginning of acompression cycle, an air pocket is present inside of the divider 734with the air/liquid interface 748 just above the lower edge of thedivider side wall 752, as shown in FIG. 5A. As additional liquid isintroduced into the volume of the pressure vessel, the air/liquidinterface 748 moves upward as the additional liquid drives theair/liquid interface 748 toward the divider upper wall 746 andcompresses air within the volume of the pressure vessel 726. The processcontinues until the air/liquid interface 748 eventually reaches thepassages 750 between the divider 734 and manifold 736, and liquid beginsto enter the manifold 736 itself, as shown in FIG. 5B. Eventually, nearthe end of the compression cycle, the air/liquid interface 748 maycontact the upper wall 746 of the divider 734, as shown in FIG. 5C, andnearly or completely fill the manifold 736.

According to some embodiments, the area of the air/liquid interface of adivider may remain substantially constant, at least until the air/liquidinterface reaches the top of the upper wall, due to a substantiallyconstant cross-sectional area between sidewalls of a divider, althoughthere may be some insubstantial change in air/liquid interface area dueto flaring and/or a reduction in area of air-exposed side wall as theair/liquid interface moves higher within a pocket of air. A relativelyconstant, relatively high air/liquid interface area throughout thecompression process may help promote heat transfer from the airthroughout the compression process.

According to some embodiments, features may be included in acompressor/expander device to balance the flow of air and/or liquidbetween a manifold and pockets of air under the dividers of a pressurevessel. The flow may be balanced such that the air/liquid interface ofeach of the dividers of a pressure vessel, or some portion of thedividers of a pressure vessel, may move within dividers synchronously,such as to reach upper walls of the dividers at a common time. In thisrespect, areas for heat transfer between air at the air/liquid interfaceand at the air/divider interface may be maintained in each of thedividers throughout a compression and/or expansion process. In someembodiments, ports between the manifold and each of the dividers may besized differently to accomplish balanced flow. Additionally and/oralternatively, ports between the manifold and dividers may includevalves to provide balanced flow. The ports and/or valves may beconfigured to account for the gravitationally induced pressure gradientin the pressure vessel. For example, the ports near the bottom of thepressure vessel may be sized smaller than the ports near the top of thepressure vessel in order to accommodate the higher pressures expected atthe bottom of the pressure vessel.

During an expansion mode, the air/liquid interface moves in the dividersof a pressure vessel essentially in the opposite direction as duringcompression. For instance, the expansion process may begin with thepressure vessel, including the manifold and dividers, entirely orsubstantially filled with liquid. Air forced into the port of thepressure vessel may move liquid downwardly through the manifold, asshown in FIG. 5C, eventually passing through the passages and enteringeach of the dividers, creating pockets of air and air/liquid interfacestherein. As air continues to expand into the volume, the air/liquidinterface of each divider may move lower, as shown in FIG. 5B,eventually reaching a level just above the lower edge of the dividers,as shown in FIG. 5A. Any air/liquid interface that happens to pass belowthe lower end of a divider side wall may cause air to pass between theinner pressure vessel walls and the outer walls of the dividers. Thisair may eventually reach the top of the pressure vessel and re-enter themanifold through passages near the top of the pressure vessel or throughanother mechanism included for this purpose. In another embodiment, airforced into the port of the pressure vessel may move liquid downwardlypast the dividers, without creating pockets of air. In thisconfiguration, the pressure vessel holds only one pocket of air, and theair volume grows larger during the expansion process until the expansionstroke is concluded.

Similar to compression, the overall air/liquid interface area may remainsubstantially constant throughout the expansion process, at least aftermoving away from the upper surface of each divider and before theair/liquid interface moves below the lower edge of any divider. In otherconfigurations, the air/divider interface will increase linearly orgeometrically through the expansion process.

Using liquid in a pressure vessel to compress and displace air mayprovide several benefits. According to some embodiments, the liquid mayact as a water piston that conforms to the shape of a pressure vesselwhen used to compress and displace air therefrom. According to someembodiments, the water piston may essentially occupy the entire volumeof the pressure vessel, thus eliminating any clearance volume. Usingwater as the positive displacement mechanism also provides a heatmanagement mechanism, thus serving multiple purposes. Additionallyand/or alternatively, in some embodiments, excess liquid may beintroduced to the pressure vessel as liquid condenses out of air that iscompressed. Condensed liquid may be combined with liquid that resides inthe pressure vessel without adverse effects. It is possible, accordingto some embodiments, that enough liquid may condense to cause the totalvolume of liquid to exceed the volume available in a pressure vessel atsome points during the operating cycle of a compressor/expander device.In such scenarios, excess liquid may exit the pressure vessel throughthe port or through another mechanism included for this purpose, withoutadverse effect, along with air that is being compressed and displaced.Excess liquid may be removed through moisture traps, or by means knownto those of skill in the art. Any liquid deposited into the pressurevessel during compression is removed and retained at minimal loss, usingan intermediate reservoir, to a holding tank. During expansion, liquidcan be vaporized, thereby removing liquid from the pressure vessel.Liquid held in the holding tank can be re-injected during expansion soas to maintain the total liquid volume in the system constant. In thismanner the compression/expansion system does not consume any liquid.

Using water as the positive displacement mechanism also provides a nearzero friction piston seal, and a zero leakage piston seal, which reducesenergy losses due to friction, reduces maintenance and inefficiency dueto seal wear, eliminates the need to replace the piston seal, improvingdevice and process reliability. It also eliminates the need to lubricatethe piston on the cylinder or to maintain, service, and replace thelubricant or its filter and/or filtering system, or to cool thelubricant, and to avoid the energy losses associated with pumping,filtering, and cooling the lubricant.

Liquid within a pressure vessel, according to some embodiments, may alsoact in combination with a heat exchanger to transfer heat from air thatis compressed (or to air that is expanded) to an external environment(or from an external environment). By way of example, FIG. 6 shows aheat exchanger 854 that extends through a wall of the pressure vessel826 to contact both the liquid and the external environment. Asillustrated, the heat exchanger may include a circular array of heatpipes, although other types of heat exchangers may be used, additionallyor alternatively. As is to be appreciated, heat pipes operate with arefrigerant that evaporates at one end of the pipe where heat isreceived, and that condenses at the other end of the pipe where heat isremoved, approximately at the same temperature as that which heat isreceived, or within a small range of temperatures, such as ranges ofabout 4° C. It is to be appreciated that FIG. 6 shows but one heat pipearrangement that may be used to transfer heat to or from liquid of apressure vessel, and that other arrangements may also exist, such asarrangements that include heat pipes or other types of heat exchangespositioned in actuator housings or other components that are in fluidcommunication with a pressure vessel. According to another embodiment,heat pipes may be provided in direct contact with the dividers, some ofwhich may trap air pockets within a pressure vessel. It is also to beappreciated that any heat source or heat sink may be used in theenvironment external to the pressure vessel to provide or receive heattherefrom, as embodiments of the system are not limited to any onearrangement of heat sources or heat sinks.

The use of a liquid as a medium through which heat passes duringcompression and/or expansion may allow for a continuous cooling process.That is, during compression the liquid may receive heat from air that isbeing compressed, and pass this heat to the external environmentcontinuously, both while air is being compressed and while air is beingreceived by the pressure vessel for later compression. Similarly, heataddition may occur when a compressor/expander device is operating in anexpansion mode both during expansion and as expanded air is passed froma pressure vessel.

According to some embodiments, the liquid in the compressor/expanderdevice may include water, although other liquids may be used,additionally or alternatively. As is to be appreciated, water maynaturally condense out of air that is being compressed by the system,and in this respect, may combine with the liquid without adverse impact.Additionally, when used in embodiments of the expander/compressordevice, water may evaporate into air during expansion without having anadverse impact. Other types of liquids, however, may be used in additionto or in place of water. Some examples of such liquids may includeadditives or entire liquids formulated to prevent freezing, such asglycol, liquids that prevent evaporation, such as glycerin, liquids toprevent corrosion, liquids to control viscosity, liquids to controlthermal conductivity, liquids to control lubricity, liquids to preventbiological agents from growing, liquids to adhere to surfaces of thepressure vessel, liquids to enhance the operation of the valves in thesystem, liquids to handle the build-up of any minerals such as salt froma salt cavern, and/or liquids to prevent foaming.

One embodiment may use a phase change material as thecompression/expansion medium directly in the pressure chamber. In thisway, the liquid not only provides the surface with which air iscompressed, but also serves as a heat transfer mechanism. A liquidundergoing a phase change (whether to or from gas or solid phases)remains at constant temperature. This can be taken advantage of withinthe pressure vessel to keep the expansion or compression temperatureisothermal by direct means, without requiring a heat exchange device.Heat transfer occurs by direct contact between the air and phase changeliquid. This heat transfer mechanism can be implemented in a variety oftechniques apparent to the artisan, including contacting the air with aspray or mist of the working liquid (such as water), using a workingliquid that boils at a suitable temperature and the vapor phase of whichcan be readily separated from the air after compression and beforestorage (e.g. by condensation), and/or using working liquid that freezesat a suitable temperature (e.g. by operating the system at conditions inwhich the working liquid is a mixture of the liquid and its solid form,such as a water ice slush).

Compressor/expander devices may be arranged in series to create amulti-stage compressor/expander device, according to some illustrativeembodiments. FIGS. 7A-7I illustrate an example of a multi-stagecompressor/expander device including three-stages. Each of the first,second, and third stages comprise a pair of pressure vessels, similar tothe pressure vessels described with respect to FIG. 3A, connected influid communication to an actuator. In other configurations, there couldbe one, three, four, or more pressure vessels in each stage.Specifically, an actuator for the first stage includes a housing orconduit 940 disposed between a first pressure vessel 926 and a secondpressure vessel 928, an actuator for the second stage includes a housing940′ disposed between a first pressure vessel 926′ and a second pressurevessel 928′, and an actuator 940″ for the third stage includes a housing940″ disposed between a first pressure vessel 926″ and a second pressurevessel 928″. A piston 932, 932′, 932″ is movably disposed within thehousing 940, housing 940′ and housing 940″, respectively. Multipledividers 934 are disposed within each of the first pressure vessel 926and the second pressure vessel 928 of the first stage, multiple dividers934′ are disposed within each of the first pressure vessel 926′ and thesecond pressure vessel 928′ of the second stage, and multiple dividers934″ are disposed within each of the first pressure vessel 926″ and thesecond pressure vessel 928″ of the third stage, as shown, for example,in FIGS. 7B-7I.

The first and second pressure vessels 926 and 928, respectively, of thefirst stage each include a first valve 956 that opens to allow thereceipt of air from the environment. These valves, and those referencedbelow, may be actively controlled, passively controlled, or may be anactive or passive port. Each of the first and second pressure vessels926, 928 of the first stage is also fluidly coupled to a pressure vessel(926′, 928′) of the second stage by a conduit 958, 960 that may includeone or more second valves 962 to selectively open and close fluidcommunication between the volumes of the corresponding pressure vessels.The pressure vessels 926′, 928′ of the second stage are also fluidlycoupled to pressure vessels 926″, 928″ of the third stage throughconduits 964, 966, and include one or more third valves 968, 968′ thatselectively open and close fluid communication therebetween. Fourthvalves 970 are additionally placed downstream to the ports of pressurevessels 926″, 928″ at the third stage to control the passage of airbetween the third stage and a storage structure (not shown) to or fromwhich pressurized air is passed. It is to be appreciated that, althoughdescribed herein as a three-stage compressor/expander device, fewer oradditional pressure vessels and/or valves can be included to createfewer or additional stages of compression/expansion.

According to one illustrative embodiment, constructed similarly to thatshown in FIG. 7A, the first stage may be configured to provide acompression ratio of about 10.14:1, the second stage has a compressionratio of about 5.5:1, and the third stage of has a compression ratio ofabout 3.3:1. Such compression ratios may be suitable for a system thatis configured to compress air to a pressure of about 184 atmospheresfrom a starting pressure of about atmospheric pressure, and to expandair from 184 atmospheres to about atmospheric pressure. Acompressor/expander device configured in this manner may have a powerrating of about 2 megawatts, according to one embodiment. In anotherembodiment, the stages may have roughly equivalent pressure ratios ofaround 5, 6, 7, or some other number. In another embodiment, a separatecompression and/or expansion device or process such as a screwcompressor and/or expander, centrifugal compressor and/or expander,bellows compressor and/or expander, piston compressor and/or expander,or other compressor and/or expander device or process may providecompression and/or expansion for the first stage, the second stage, thethird stage, or some combination of stages, at a pressure ratio of 2:1,3:1, 4:1, 5:1, 6:1, or some other number.

In the embodiment of FIG. 7A, a compression cycle may begin with thepiston 932 of the actuator for the first stage moving away from thefirst pressure vessel 926 of the first stage to increase the volumeavailable for air inside of the first pressure vessel 926 of the firststage, as represented by FIG. 7B. This movement may pull water out ofthe dividers 934 of the first pressure vessel 926, creating negativepressure that draws ambient air into the first pressure vessel 926 intopockets within each of the dividers 934, creating additional air/liquidand air/divider interfaces through which heat may be transferred. Inanother embodiment, this movement may pull water out of the dividers 934of the first pressure vessel 926, creating negative pressure that drawsambient air into the first pressure vessel 926 into and through or pastthe dividers 934, creating additional air/liquid and air/dividerinterfaces through which heat may be transferred. When the piston 932reaches the end of this stroke, the first valve 956 between theatmosphere and the first pressure vessel 926 is closed and the secondvalve 962 between the first stage and second stage is opened, as shownin FIG. 7C. The compression stroke begins as the piston 932 returnstoward the first pressure vessel 926, decreasing the volume availablefor air in the combined volume of the first pressure vessel 926 of thefirst stage and the first pressure vessel 926′ of the second stage,compressing and displacing the air toward the first pressure vessel 926′of the second stage, as shown in FIG. 7D. In this respect, compressionof air may take place across pressure vessels of different stages. Asthe piston 932 of the first stage nears the end of its stroke toward thefirst pressure vessel 926, the piston 932′ of the second stage nears theend of its stroke away from the first pressure vessel 926′ of the secondstage and the second valve 956 between the first pressure vessel 926 ofthe first stage and the first pressure vessel 926′ of the second stagecloses, as shown in FIG. 7E. Operation between the second and thirdstages of the compressor/expander device mirrors the above describedoperation between the first and second stages. Operation between thethird stage and the storage structure, however, may differ in that thevalve (e.g., valve 970) to the storage structure may open when thepressure at the third stage exceeds the pressure of air in the storagestructure, rather than when the piston of the third stage begins itscompression stroke.

The above described compression cycle differs from existing positivedisplacement compression cycles, in that the compression in the pressurevessels includes the air volume of pressure vessels of multiple stages,rather than that of a single stage. In contrast, prior art compressorstypically compress air in a single compression chamber (i.e., pressurevessel). It is to be appreciated other embodiments could be implementedwith any number of pressure vessels at a common stage, or spread amongmultiple stages, as the various embodiments are not limited to thatdescribed herein. Additionally, in embodiments where compression occursacross pressure vessels of different stages, volumetric ratios of anystage may be modified by adjusting valve timing between various stages.

An expansion cycle, in an embodiment constructed like that of FIG. 7A,is represented in FIGS. 7F-7I. Expansion begins with air expanding intoa first pressure vessel 926″ of the third stage from a storage structure(not shown), as represented in FIG. 7F. This expanding air moves liquidto drive the third actuator (e.g. piston 932″) away from the firstpressure vessel 926″ of the third stage. This process continues, withthe third valve 968′ to the first pressure vessel 926′ of the secondstage closed, as shown in FIG. 7G. As the third piston 932″ nears theend of travel away from the first pressure vessel 926″ of the thirdstage, as shown in FIG. 7H, the fourth valve 970 closes fluidcommunication with the cavern (e.g., storage structure). The third valve968′ is then opened to allow the air to expand into the first pressurevessel 926′ of the second stage, driving the second actuator (e.g.,piston 932′), as shown in FIG. 7I. Operation between the third andsecond stages, and then the second and first stage of thecompressor/expander device mirrors the above-described operation betweenthe storage structure and the third stage.

Compressor/expander devices may be installed modularly, allowing systemsto be constructed for a wide range of energy storage needs. By way ofexample, a compressor/expander device, such as the device of FIG. 7A,may be sized to store and generate between 1.0 and 5.0 megawatts ofpower, although it is to be appreciated, that other embodiments mayinclude higher or lower power ratings. A plurality ofcompressor/expander devices may be installed together and operated inparallel for installations having higher energy storage powerrequirements, for instance as high as 300 megawatts or higher, accordingto some embodiments. Installations that include multiplecompressor/expander devices, arranged in parallel, may operate at lessthan full capacity by shutting down a portion of the compressor/expanderdevices, or by operating some or all of the compression/expansiondevices at less than their full power capacity, which may promoteefficient system operation. Installations that include multiplecompressor/expander devices, arranged in parallel, may operate at morethan full rated capacity for some duration in order to meet a particularoperating requirement, such as compressing air when power prices are lowor negative, or expanding air when power prices are high. Suchoperations may be affected by increasing the speed of the compressionstroke, increasing the mass of air in the intake of the third stage fromthe storage vessel by controlling the timing of the valves in thesystem, particularly the valve(s) between the storage vessel and thethird stage. Additionally or alternatively, installations that includemultiple compressor/expander devices may be constructed modularly toallow system operation before all compressor/expanders are installed, orduring periods of time when one or more compressor/expanders are downfor maintenance, repair, replacement, or for other reasons. Additionallyor alternatively, installations that include multiplecompressor/expander devices may be constructed modularly to allow morecompressors/expanders (the spares) to be constructed than called for bythe power rating of the project, allowing the spares to take over justas or shortly after various compressor/expander units are turned off formaintenance, repair, replacement, or other reasons, thereby maintaininga higher power rating for the power plant.

Embodiments of the compressor/expander device can accommodate a widerange of operating power levels. As is to be appreciated, it may bedesirable to store or release energy at varying rates, particularly whenenergy to be stored is received from a less predictable source, such asa wind farm. A compressor/expander device described herein can act as apositive displacement device, meaning that the overall device intakes acommon volume of air during each cycle, although each stage compressesthis initial volume to different values. Such positive displacementdevices may operate at different power levels by compressing (orexpanding) different masses of air that have a common volume, unlikecentrifugal compressors typically used in CAES systems that operateefficiently primarily at a relative narrow range of power levels.Additionally or alternatively, installations having a plurality ofcompressor/expander devices that operate in parallel may activate only aportion of the installed compressor/expander devices to accommodatedifferent operating power levels.

The compressor/expander devices may operate at relatively slow speeds,which may provide for improved heat transfer, improved energyconsumption and/or generation, improved durability, decreased entropylosses, decreased pressure drops through valves, pipes, and ports,decreased thermal cycling of the compressor/expander, and/or improvedreliability. According to some embodiments, a compression or expansioncycle of the compressor/expander device may allow for improved heattransfer, which may allow the device to achieve near isothermal behaviorduring expansion and/or compression. Additionally, lower temperaturesassociated with improved heat transfer and less friction at joints andsliding contacts in the expander/compressor device may provide forimproved durability and reliability, as compared to higher speedmachinery.

Slower operating speeds and/or increased heat transfer capacities ofvarious embodiments of the compressor/expander device enable heattransfer to occur with the external environment across relatively lowtemperature differences. According to some embodiments, theexpander/compressor device may operate with near isothermalcompression/expansion processes while exchanging heat with the externalenvironment across temperature differences as low as 50° C., as low as25° C., or even as low as 5° C.

According to some embodiments, low-grade heat sources and/or heat sinksmay be used to provide heat to and receive heat from thecompressor/expander during expansion/compression modes. In this respect,the system may be capable of operating without burning fossil fuels,such as for heating air at expansion. It is to be appreciated, however,that embodiments of the system may also be operated in conjunction withpower plants or other systems that do burn fossil fuels. Someembodiments may use geothermal energy, solar energy, and other energyinputs, water, salt water, gravel, water and gravel, salt water andgravel, and other thermal heat sinks and sources as heat sources and/orsinks, taking advantage of the substantially constant groundtemperatures that exist 4 to 10 meters below the earth's surface andsubstantially constant temperatures associated with subterraneancaverns, when used as storage structures. Additionally, according tosome embodiments, compression may occur at night when the airtemperature is lower and may provide an environment to which heat isremoved while expansion occurs during the day when temperatures arehigher and may provide a source of heat used in the expansion process.

According to some embodiments, a system that utilizes acompressor/expander device may have a modular construction. By way ofexample, FIG. 8 shows one embodiment of a compressor/expander device1020 that is incorporated directly into the structure of a wind turbine1014. The wind turbine 1004 includes a rotor 1072 that is connected toand drives a low speed hydraulic pump 1074 through a gearbox 1076. Thegearbox 1076 may be a mechanical gearbox, a hydraulic gearbox, or mayinclude other types of gearboxes. A conduit 1078 connects a hydraulicfluid output of the hydraulic pump 1074 to a hydraulic motor 1080 thatis mechanically connected to a generator 1082. The conduit 1078 alsoconnects the hydraulic fluid output of the hydraulic pump 1074 to one ormore actuators of a compressor/expander device 1020, that may bepositioned in the tower 1016 of the wind turbine 1014. Each of the gearbox 1076, hydraulic pump 1074, hydraulic motor 1080, and generator 1082are shown positioned in the nacelle 1018 of the wind turbine 1014, butcould be positioned elsewhere in other embodiments. One or more valves1084 may control the flow of hydraulic fluid from the hydraulic pump1074 to the hydraulic motor 1080 and/or compressor/expander device 1020,according to a mode in which the system is operating. The wind turbine1014 also includes a storage structure 1086 that may be located inportions of the tower of the wind turbine 1014 and/or a storagestructure 1022 in a foundation 1088 that supports the wind turbine 1014.In this respect, the wind turbine may provide for a self-containedenergy storage and retrieval system that may prove beneficial foroffshore applications.

The system of FIG. 8 may operate in different modes. In a first mode ofoperation, wind energy may be directed solely to the generator 1082 ofthe wind turbine 1014. In this mode, one or more valves 1084 may bepositioned so that hydraulic power does not go to thecompressor/expander device 1020, such that any power associated withwind driving the rotor 1072 is converted through the gear box 1076, thehydraulic pump 1074, the hydraulic motor 1080, and the generator 1082into electricity. In a second mode of operation, wind energy may be usedexclusively to drive the compressor/expander device 1020 to store energyas compressed air. In this mode, the one or more valves 1084 may bepositioned such that hydraulic power is directed solely to thecompressor/expander device 1020 from the hydraulic pump 1074. The valves1084 may also be positioned such that hydraulic fluid from the hydraulicpump 1074 goes to the compressor/expander device 1020 and thecombination of the hydraulic motor 1080 and generator 1082, such thatwind energy may be used to compress air and to create electricity at acommon time. When it is desirable to release energy that is stored inthe system, compressed air may be released for expansion through thecompressor/expander device 1020 in yet another mode of operation.Pressurized hydraulic fluid, output from the compressor/expander device1020, may drive the generator 1082, through the hydraulic motor 1080, tocreate electric energy. This may occur either to assist the hydraulicpump 1074 that is being driven by the rotor 1072 when there is adequatewind, or as a sole source of pressurized hydraulic fluid, when there isinadequate wind to rotate the rotor 1072.

According to some embodiments, a system that is incorporated into thestructure of a wind turbine may share components with the wind turbineitself, realizing additional and/or alternative efficiencies. By way ofexample, a compressor/expander device may utilize control softwarenormally dedicated to the wind turbine or otherwise share controlsoftware and/or hardware with the wind turbine. The generator, gearbox,hydraulic pump, valves, and/or hydraulic motor may be common to both thecompressor/expander device and the wind turbine to reduce the cost andnumber of components used in a system.

FIG. 9 shows a cross-sectional, schematic view of one embodiment of acompressor/expander device 1120 that may prove suitable for packagingwithin the tower of a wind turbine. As shown, first and second pressurevessels 1126 and 1128 are positioned vertically with respect to oneanother. The first pressure vessel 1126 includes dividers 1134 and amanifold 1136, and the second pressure vessel 1128 includes dividers1134′ and a manifold 1136′. The first and second pressure vessels 1126,1128 are connected by a hydraulic actuator 112 and a housing 1140 thatis wider in diameter than each of the pressure vessels 1126, 1128. Ahydraulically actuated piston 1132 is disposed within the housing 1140.The larger width of the housing 1140, relative to the pressure vessels1126, 1128, may reduce distances and, correspondingly, velocitiestraveled by liquid at a given operating speed. Reduced liquid velocitiesmay, in turn, reduce liquid pumping resistance within thecompressor/expander device 1120 to help improve the compressor/expanderdevice 1120 operating efficiencies.

A compressor/expander device, according to some embodiments, may operateat a substantially constant output power when in an expansion mode forvarying storage structure air pressure levels. FIG. 10 is a graphshowing air pressures through each of three stages of acompressor/expander device for two storage structure pressure levels,according to one embodiment. As illustrated, air pressures throughoutthe expander devices may follow a similar but shifted curve afterinitial expansion to produce a similar amount of power for storagestructure air pressures between 100 and 180 bar, represented by thedotted line in FIG. 10. It is understood that other pressure ranges,however, may alternatively be employed. Sensors, valves, controllers andother devices may be used to control a mass of air that enters thecompressor/expander device from the storage structure to accomplishthis. In one embodiment the final discharge pressure may be higher thanambient air pressure.

In another embodiment, the vessels and pumps are sized to admit agreater and adjustable volume of air during expansion than compressionenabling them to generate the full rated power during expansion from thelowest storage pressure. A vessel/pump system designed according to thisembodiment will only be fully utilized during expansion from the lowestdesign storage pressure. Further, a vessel/pump system designedaccording to this embodiment will always be fractionally used duringcompression. In another embodiment the final discharge pressure mayapproach ambient air pressure.

In another embodiment, regenerative heat exchange techniques can be usedto extract heat energy from the air during compression (e.g. via theworking liquid and/or the dividers) and to insert heat energy into theair during expansion (again, e.g. via the working liquid and/or thedividers). This functionality can be implemented using any of a varietyof techniques that will be apparent to the artisan. For example, aregenerative heat exchange system can include a heat exchanger inthermal communication with the compressor/expander device (e.g. thatcirculates a suitable thermal working fluid through a heat exchanger,the other side of which is exposed directly to the air or working fluidin the compressor/expander device, or indirectly via the dividers orother intermediary heat transfer structure) and a heat energy storagereservoir (e.g. a an insulated storage tank for the thermal workingfluid). During compression, the regenerative heat exchange system can beoperated to circulate the thermal working fluid to extract heat energyfrom the air and to insert that heat energy into the storage reservoir.Conversely, during expansion, the regenerative heat exchange system canbe operated to circulate the thermal working fluid to extract heatenergy from the reservoir and to insert that heat energy into the air.

Heat removal from air that is being compressed and/or heat addition toair that is being expanded may help minimize temperature changes thatoccur in the air during these processes and, as described herein, mayhelp a system achieve process conditions that are isothermal, oracceptably close to isothermal to be economically optimal. For example,as used herein, “isothermal” or “near isothermal” can means that theheat transfer process is characterized by a polytropic index of about1.1 or less, and preferably about 1.05 or less. According to oneembodiment, air experiences less than a change in temperature of about1.6° C. or less throughout compression and/or expansion processes in acompressor/expander device (corresponding to a polytropic constant of1.023). It is to be understood, however, that the system may also beoperated in configurations that implement compression/expansionprocesses corresponding to a polytropic index greater than 1.1. Forexample, the equipment and/or operating costs required to operate asystem so as to achieve a polytropic index of 1.05 may exceed the costsof the thermal inefficiencies in implementing an operating a system thatachieves a polytropic index of greater than 1.1. It may therefore bedesirable to implement the system so as to operate at the higherpolytropic index.

Embodiments of the compressor expander device may be configured to reachoperating speeds and/or power levels quickly to provide ancillaryservices to power facilities, including but not limited to, black startservices, spinning reserve services, voltage support services, and/orfrequency regulation services.

It is to be appreciated that, although described herein primarily foruse with wind turbines and/or wind farms, embodiments of thecompressor/expander device may be used with various types of powerproduction facilities, including but not limited to solar power plants,coal fired power plants, gas fired power plants, nuclear power plants,geothermal power plants, biomass power plants, and/or hydro powerplants, to name a few. In one embodiment, the thermal energy from asolar plant would be used through the device and process describedherein rather than or in addition to a more traditional steam turbine ororganic Rankine Cycle turbine, or other heat engine, as part or all ofthe heat source to improve the efficiency of generating power whenexpanding compressed air. The conversion efficiency of thermal energy toelectric power from the solar plant may be 70%, 80%, and higher throughthis system.

Although the embodiments of a compressor/expander device are describedherein for use in compressing or expanding air, it is to be appreciatedthat a compressor/expander device may be used to compress and/or expandany other gaseous substance, such as, but not limited to carbon dioxide,natural gas, oxygen, nitrogen, butane, propane, and other gasses. It isalso to be appreciated that embodiments of the compressor/expanderdevice are described herein for use with water or liquid, and that anyother liquid-like substances that may also be used as a heat transferand/or pressure transfer medium, including other types of coolants.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

In some embodiments, a device as described herein includes at least onepressure vessel in which air may be compressed and/or expanded. The atleast one pressure vessel is at least partially filed with liquid and,at times, with air. The at least one pressure vessel is coupled to anactuator that moves the liquid in the volume to compress air in thepressure vessel or that is moved by air that is expanded within thepressure vessel to drive the actuator. The pressure vessel includes aplurality of dividers that hold air and/or are located throughout thepressure vessel so as to create a high area for thermal conductionto/from the air, the dividers, and the liquid. The dividers maysubstantially reduce the aggregate length of the heat path between airand liquid or structure, as compared to similarly constructed vesselsthat lack dividers. The dividers may substantially increase the totalarea of the air/liquid interface, as compared to similarly constructedvessels that lack dividers. Additionally, the dividers may provide anair/liquid interface and/or air/divider interface and/or liquid/dividerinterface that remains substantially constant in area throughout andtoward the end of a compression or expansion cycle, where airtemperature changes, absent heat transfer, might otherwise be thegreatest. In some embodiments, the dividers may be arranged in a stackconfiguration with each divider in fluid communication with a manifoldthat, in turn, is in fluid communication with a port of the pressurevessel.

In some embodiments, a device as described herein can compress and/orexpand air and includes two or more stages, arranged in series, thateach include a first pressure vessel and a second pressure vessel and anactuator that is coupled to each of the first pressure vessel and thesecond pressure vessel. The volume of each of the first pressure vesseland second pressure vessel is at least partially filed with liquid thatis moved within a corresponding pressure vessel by the actuator toalternately compress air and allow for the expansion of air in a portionof the corresponding volume that is not occupied by liquid. The actuatormoves between the first pressure vessel and the second pressure vesselsuch that each of the first pressure vessel and the second pressurevessel are acting out of phase with one another. Actuators of each ofthe two or more stages of the device move out of phase with respect toactuators of any immediately upstream and/or downstream stages.According to some embodiments, dividers may be included in each of thepressure vessels to increase the area available for heat transfer to orfrom air that is being compressed and/or expanded.

In some embodiments, a device as described herein can compress and/orexpand air isothermally or near isothermally. The device includes apressure vessel at least partially filled with liquid. The pressurevessel is connected to an actuator that may move the liquid in thepressure vessel to compress air therein, or that may be moved by liquidthat is displaced as air expands in the pressure vessel. The liquid isin contact with the air at one or more air/liquid interfaces andair/divider interfaces and liquid/divider interfaces, across which heatis transferred from air that is compressed and/or to air that isexpanded. The pressure vessel also includes a heat exchanger, such asone or more heat pipes, that transfers heat between the liquid and anenvironment that is external to the device. Heat may be moved from airthat is compressed and/or to air that is expanded to achieve isothermalor near isothermal compression and/or expansion processes. A relativelytotal heat transfer surface area (i.e., air/liquid interfaces andair/divider interfaces and liquid/divider interfaces) and/or relativelyslow cycle speeds (e.g., 6 seconds for a single compression or expansioncycle) may help the device achieve isothermal or near isothermalcompression and/or expansion.

In some embodiments, a plurality of devices as described herein can eachcause energy to be stored by compressing air for storage and laterrelease the compressed air, through the same plurality of devices, forexpansion and the production of energy. Each of the plurality of devicesare sized (e.g. less than 2 megawatts of capacity or less then 1.2megawatts of capacity) such that typical installations, having powerstorage requirements 5 times, 10 times, 20 times, 50 times, 100 times,or 150 times, or even greater than the power storage capacity of asingle device, may utilize any desirable number of devices, for example,up to 5 devices, up to 10 devices, up to 20 devices, up to 50 devices,up to 100 devices, up to 150 devices, or even greater numbers of devicesto meet the power storage requirements at a particular installation.Having a plurality of devices may provide one or more benefits,including but not limited to the ability to easily store and/or releaseenergy at rates associated with less than the full power storagecapacity of an installation, the ability to remove individual devices orgroups of devices from use for maintenance or repair withoutsubstantially impacting the operation of the overall installation,and/or the ability to construct individual devices at large volumes,realizing economies of scale and facilitating shipment and installationof devices.

In some embodiments, a device as described herein can compress air forthe storage of energy as compressed air in a storage structure. Thestorage structure may hold air at varying pressure levels. The devicealso receives air from the storage structure, at varying pressurelevels, and expands the air to release energy therefrom for theproduction of electric energy. According to some embodiments, the deviceis a positive displacement compressor and/or expander that may beoperated in an expansion mode to expand air received at pressures levelsthat vary upward to as high as 250 atmospheres or more. According tosome embodiments, the device may include multiple stages, arranged inseries, for the compression and/or expansion of air.

In some embodiments, a device as described herein can compress air forthe storage of energy as compressed air in a storage structure, and canexpand pressurized air received from the storage structure to produceelectric energy, when needed. Air may be stored in the storage structureat varying pressure levels. The device includes a pressure vessel and anactuator that moves liquid through the device across a substantiallyconstant swept volume. The device includes valves and a controller thatmay be programmed to control a mass of air that is received by thedevice for expansion regardless of the pressure at which air is held inthe storage structure.

In some embodiments, a device as described herein can compress air forthe storage of energy as compressed air, and expand the compressed air,when needed, to produce energy. The device may be incorporated into thestructure of wind turbine, such as in a nacelle or in the towerstructure. According to some embodiments, the wind turbine and thedevice may share componentry to reduce overall system costs and/orreduce overall system size, which may prove particularly useful foroffshore applications. By way of example, control software and/orhardware may be shared between the wind turbine and the device.Additionally or alternatively, a generator may be coupled both to arotor of the wind turbine through a hydraulic motor and pump, and may bedriven by the wind turbine when wind is adequate, and or by the devicewhen compressed air when wind is not adequate.

In some embodiments, a device as described herein includes an upstreampressure vessel and a downstream pressure vessel in which air may becompressed. Each of the upstream pressure vessel and the downstreampressure vessel is at least partially filled with liquid and, at times,with air. A maximum volume available for air in the downstream pressurevessel is less than a maximum volume available for air in the upstreampressure vessel. The upstream pressure vessel is coupled to an upstreamactuator and the downstream pressure vessel is coupled to a downstreamactuator. Each of the upstream actuator and the downstream actuator moveliquid in an internal volume of the corresponding pressure vessel toalternately increase and decrease a volume available for air in thecorresponding pressure vessel. A conduit extends between the upstreampressure vessel and the downstream pressure vessel and includes a valvethat may be selectively opened to provide fluid communication betweenthe upstream pressure vessel and the downstream pressure vessel.Compression of air begins with the valve open to provide fluidcommunication between the upstream pressure vessel and the downstreampressure vessel. At the beginning of compression, the volume availablefor air in the upstream pressure vessel is at the maximum value and thevolume available for air in the downstream pressure vessel is at aminimum value. The upstream actuator then moves liquid in the upstreampressure vessel to compress air in the volume available for air of theupstream pressure vessel, the conduit, and the volume available for airin the downstream pressure vessel. Simultaneously, the downstreamactuator moves liquid in the downstream pressure vessel to increase thevolume available for air in the downstream pressure vessel. Air iscompressed in each of the volume available for air of the upstreampressure vessel, the conduit, and the volume available for air in thedownstream pressure vessels as the magnitude of the decrease in thevolume available for air in the upstream pressure vessel is greater thanthe magnitude of the increase in the volume available for air of thedownstream pressure vessel.

While various embodiments of the invention have been described above, itshould be understood that they have been presented by way of exampleonly, and not limitation. Where methods and steps described aboveindicate certain events occurring in certain order, those of ordinaryskill in the art having the benefit of this disclosure would recognizethat the ordering of certain steps may be modified and that suchmodifications are in accordance with the variations of the invention.Additionally, certain of the steps may be performed concurrently in aparallel process when possible, as well as performed sequentially asdescribed above. The embodiments have been particularly shown anddescribed, but it will be understood that various changes in form anddetails may be made.

For example, although various embodiments have been described as havingparticular features and/or combinations of components, other embodimentsare possible having any combination or sub-combination of any featuresand/or components from any of the embodiments described herein. Thespecific configurations of the various components can also be varied.For example, the size and specific shape of the various components canbe different than the embodiments shown, while still providing thefunctions as described herein.

The compressor/expander units may be arranged modularly in a project,and they may be placed outside or inside a building. In the building,they may be arranged in a configuration with a central aisle, with theunits adjacent to each other on either side of the aisle. Thecompressor/expander units may be interconnected with each other withsome or all of the following: electricity, water, hydraulic fluid, air,lubricating oil, hot water, cold water, and other common services. Theremay be separate stores and/or sources of hot and/or cold water for thecompressor/expanders.

1. An apparatus for use in a compressed gas-based energy storage andrecovery system, the apparatus comprising: a pressure vessel defining aninterior region in which at least one of a liquid or a gas can becontained; a plurality of downwardly opening dividers arranged in astack, the stack positioned in the interior region of the pressurevessel to divide the interior region into a plurality of sub-regions;and a manifold extending through the stack of dividers and in fluidcommunication with the plurality of sub-regions.
 2. The apparatus ofclaim 1, wherein each of the downwardly opening dividers are configuredto contain gas and provide a gas to liquid interface for transferringheat energy.
 3. The apparatus of claim 1, wherein each of the downwardlyopening dividers provide a gas to divider interface for transferringheat energy.
 4. The apparatus of claim 2, wherein the gas to liquidinterface has surface area and each of the downwardly opening dividersis configured to contain gas in contact with liquid at a substantiallyconstant gas to liquid interface surface area as liquid is moved to orfrom the interior region of the pressure vessel.
 5. The apparatus ofclaim 1, wherein each of the downwardly opening dividers include asubstantially vertical sidewall and an upper wall that form adish-shaped structure.
 6. The apparatus of claim 5, wherein the pressurevessel includes a substantially vertical sidewall, and wherein thesubstantially vertical sidewall of each of the downwardly openingdividers conforms to the substantially vertical sidewall of the pressurevessel.
 7. The apparatus of claim 1, further comprising: heat transferfins disposed on at least one of the plurality of downwardly openingdividers.
 8. An apparatus for use in a compressed gas-based energystorage and recovery system, the apparatus comprising: a pressure vesseldefining an interior region in which at least one of a liquid or a gascan be contained; a plurality of dividers positioned in the interiorregion of the pressure vessel to divide the interior region into aplurality of sub-regions, each configured to contain gas and provide agas to liquid interface, at least one of the dividers having asubstantially vertical sidewall and an upper wall that form adish-shaped structure; and a manifold having one or more aperturesadjacent to the upper wall of the divider to allow fluid communicationbetween the gas contained by the divider and the manifold.
 9. Theapparatus of claim 8, further comprising: a manifold in fluidcommunication with the gas contained by each of the dividers.
 10. Theapparatus of claim 8, wherein at least one of the dividers has a domedupper wall.
 11. The apparatus of claim 8, wherein at least one of thedividers has a flared sidewall.